Active matrix arrays, used for applications such as liquid crystal displays, are typically produced based on very strict criteria. Finished displays may be characterized by human eye or by camera to detect gross defects, but it has heretofore been difficult and/or impractical to determine defects before applying the display medium or before the array is fully fabricated. This difficulty increases the cost of the process because defective arrays may be packaged and enter the marketplace.
Notwithstanding the difficulty in implementation or impracticality of use, large area electronic testers have been employed to test active matrix arrays. Pixel defects, line defects, and area (Mura) defects may be detected on display glass before the shorting bars are removed or the liquid crystal (LC) cell is constructed. Several types of these testers are in use.
For example, an 11,520 pin tester with multiple heads is known. Pin testing allows complete curve tracing of transistor arrays. However, this type of tester is not common in production because of a risk of scratching of the display through use of the pins or probes.
Another tester, based on placing an electro-optic sheet over the display and using test vectors and a camera, is known. The sheet is made from polymer-dispersed liquid crystals (PDLC) coated with a Bragg reflector. Essentially, this temporary sheet emulates a liquid crystal display. An image of the activated sheet is then obtained. This method is used more often than pin testers for display glass testing because all types of visual defects are easily seen. Transistor characteristics, however, are not directly measurable. This method is not generally used in production because of the need to apply the PDLC sheet to the array, which may result in damage.
Still another known method uses a secondary emission electron beam. This method can probe arrays with exposed metal by using an energy analyzer to determine the static potential of known conductors. Charging and discharging of pixels may be directly observed; however, this is a very complicated and expensive process.